THE GEEBES. 225 



with a number of cells or sacs, varying greatly in size. 

 These sacs communicate with the lungs, and are filled 

 with air, this fact being evident at every respiration 

 which the bird makes. The reader will now see that 

 these air-sacs form an elastic cushion between the body 

 of the bird and the water, so that when it makes its 

 violent plunge, the shock is a very trifling one. 



Moreover, this remarkable structure serves other 

 purposes. These cells, when fully inflated, contain 

 about one hundred and eighty cubic inches of air, and 

 therefore render the bird so buoyant, that it can float 

 securely on the roughest sea. Unlike the cormorants 

 and divers, which swim very low in the water, the 

 gannet floats on the surface like a cork. 



Then it also acts as a protection against cold, and 

 enables the bird to remain at sea during weather so 

 inclement, that all other birds are driven to seek the 

 land for shelter. There are few instances in the 

 animal kingdom where a single and so simple a modi- 

 fication of structures should serve three such important 

 and yet diverse purposes as a spring- cushion, a life- 

 buoy, and a blanket. 



THE GREBES. 



There is a well-known tribe of aquatic birds called 

 the Grebes, of which we possess several indigenous 

 species, the best known and most common being the 

 Little Grebe, or Dabchick (Podiceps minor), a bird 

 which is represented on Cut 10. 



They may at once be known by the peculiar form 



15 



